A seasonal study of organic matter mineralization rates was made at 8 intertidal stations in the Westerschelde Estuary (The Netherlands). Organic matter mineralization rates, based on the gaseous emission of carbon dioxide and methane, showed significant dynamic temporal and spatial variability at various scales. Annual rates of organic matter mineralization varied from 8 to about 339 m01 C m-2 yr-l. The temperature dependence of organic matter degradation was described using an Arrhenius-type equation. Activation energies ranged from 54 to 125 kJ mol-' and correlated negatively with depth-integrated rates of mineralization. Spatial differences in mineralization were mainly due to differences in the lability of the organic matter, since the quantity of organic matter was similar between stations on an area1 or volume basis. Average first-order decomposition rate constants ranged from 0.2 to 7 yr-' and decreased towards the seaward end of the estuary due to ageing of riverinederived material. Intertidal sediments were estimated to account for about 25% of the total carbon retention in the Westerschelde estuary. KEY WORDS: Organic matter. Mineralization lntertidal sediments. Estuary. Methane .

Two approaches were used to study coupling between seagrass production and bacterial processes in the rhizosphere. In the first approach, stable carbon-isotope ratios of bacteria, sediment organic matter and plants were determined to infer sources of organic carbon used by bacteria in the sediments of 4 European Zostera marina and 2 Z. noltii meadows. Bacterial isotope ratios were derived from bacteria-specific polar lipid-derived fatty acids (PLFA), mainly methyl-branched i15:0 and a15:0. Bacterial δ 13 C ratios in the sediment from both vegetated and unvegetated sites were mostly similar and did not resemble Z. marina ratios, suggesting that seagrass material was of limited importance as a bacterial carbon source. Bacterial ratios were in most cases similar to benthic macroalgae and did correlate well with ratios of diatom biomarkers. Sediment organic matter inside the meadows had δ 13 C ratios similar to those of nearby unvegetated sites, and ratios were clearly different from the material produced by seagrasses, indicating that little seagrass material accumulated. Results from the 2 Z. noltii sites were less conclusive, as there was no clear difference in δ 13 C ratios between the potential source materials. In addition, bacterial δ 13 C ratios were highly variable at one Z. noltii site. In the second approach, cultured Z. marina was labeled with 13 C-bicarbonate to study the short-term transfer of label from plants to bacteria in the rhizosphere. However, no label was detected in bacterial PLFA after 20 h of incubation. In conclusion, a close coupling between macrophyte production and bacterial carbon cycling could not be detected in the sediment of Z. marina meadows, and benthic production by algae was probably the main carbon source for bacterial growth.KEY WORDS: Seagrass meadows · Bacterial carbon sources · Sediment · Stable carbon isotopes · Biomarkers · Zostera marina · Zostera noltii Resale or republication not permitted without written consent of the publisher

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The effect of the bacterivorous nematode Diplolaimella dievengatensis on the diffusion of oxygen into sediment and the CO, production of Spartina anglica detritus was examined in a laboratory experiment. Diffusion coefficients were calculated from measurements of both 0, consumption, using gas chromatography, and O2 micro-gradients, using micro-electrodes. After a transient state of about 10 d the diffusion and consumption of oxygen stabihzed and approach.ed steady state. In treatments with nematodes O2 consumption and CO2 production were 74 % higher than in controls. In treatments with nematodes the apparent d~ffusion coefficient of oxygen was 40 to 70 % higher than the molecular diffusion coefficient due to nematode activity. Since the increases of CO, production and of the diffusion of oxygen in the presence of nematodes were of the same magnitude, we conclude that the enhanced turnover time of Spartina detritus was largely dependent on the bioturbation activity of the nematodes.

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